It is known that the activity of catalysts gradually decreases during use, for example as a result of ageing and poisoning. To be able to maintain a constant performance level of the reaction, fresh catalyst is generally added. However, tight limits are imposed on this procedure by the progressive increase in the catalyst or solids content of the reaction suspension. For this reason, an expensive interruption of the reaction has to be carried out at regular intervals in order to discharge part or all of the generally completely or partially inactivated catalysts and replace them by fresh catalysts of the same type or a different type. The inactivated catalysts can subsequently be disposed of or used for reusing individual catalyst constituents, in particular their noble metal content, as material.
A person skilled in the art will know of various techniques by means of which a catalyst can be separated off from the reaction medium and, if appropriate, concentrated and recirculated to the synthesis:
WO 99/01206 discloses, for example, a method which separates off suspended catalysts by means of crossflow filtration over a ceramic filter. The formation of troublesome covering layers on the filters is prevented by a high flow velocity.
In CN 1394672 A and CN 1377875 A, reaction medium is taken from a chemical synthesis. The suspension medium is separated off by means of a crossflow filtration and the catalyst is recirculated with the residual suspension medium to the reactor. In this way, the catalyst suspension which is recirculated to the reactor is slowly concentrated to a maximum concentration of 12% by weight of catalyst suspension.
WO 91/16294 presents a method of recovering catalyst in the preparation of ether carboxylic acids by catalytic oxidation. The catalyst is in this case concentrated to up to 30% by weight of the catalyst suspension by means of a crossflow filtration and recirculated to the reactor. It is disclosed that a solubilizer used in the oxidation can aid the filtration, with glycol ethers without hydroxy groups being described as solubilizers.
WO 04/112957 is concerned with the recovery of a homogeneous, if appropriate suspended, catalyst by filtration over a semipermeable membrane. It is stated that monoolefinically unsaturated compounds, which are one component of the reaction medium, can be used as auxiliary in the membrane separation.
JP 5-23680 presents a process for the photooxidative treatment of water, in which a titanium dioxide catalyst is kept within a reactor by means of a filter. Water flowing through the reactor is subjected to UV radiation. The reverse osmosis membrane used for the filtration is cleaned at regular intervals by means of a reverse stream of water. Use of a plurality of these reactors in series is disclosed.
A publication by the mott-corporation, 84 Spring Lane, Farmington, USA of April 1997 describes membrane filters which can be used, either individually or in a parallel arrangement, for retaining catalysts. The catalysts are concentrated in the reaction medium and either recirculated to the reaction medium or removed from the plant during a dead-end filtration by backflushing and discharge of the concentrate. Continuous removal of catalysts is only possible by means of filters used alternately. Further treatment or conditioning of discharged, exhausted catalysts is not described.
The disposal or reuse as material of the completely or partially inactivated catalysts is carried out as a function of their composition. Catalyst suspensions which, owing to the remaining amount of toxic organic solvents or reaction products, are classified as preparations hazardous to health or the environment have to be disposed of or recycled subject to strict safety regulations and accordingly by costly processes. In the case of classification as a preparation hazardous to health or the environment, handling, transport and incineration/ashing, for example, is only possible with considerable safety precautions and/or engineering outlay.
There is accordingly great economic interest in the inactivated catalyst suspensions having only a low residual content of problematical, organic compounds, e.g. the respective organic starting materials, products or solvents. Reducing the concentration of very toxic or carcinogenic (K1+K2) substances to less than or equal to 0.1% even allows classification as preparation hazardous to health or the environment to be completely avoided (see Dangerous Materials Regulations, Directive for Preparations 1999/45/EC). However, a reduction of the residual content of problematical, organic compounds to less than 1% also results in, for example, a reduction in the classification from T+ (very toxic) to Xn (hazardous to health) and thus aids handling.
There is also considerable interest in the inactivated catalysts taken from the respective reactors being obtained directly in a state in which firstly transport and secondly reuse as material or disposal is possible without further conditioning steps or without particular safety measures. In particular, the inactivated catalyst should be free of organic compounds and be present in a nontoxic solvent, preferably water. It is also advantageous for the catalyst to be present in a high concentration.
The methods described in the prior art leave the catalyst in the reaction medium which surrounds it originally. Accordingly, the problem of freeing the catalyst of adhering and possibly hazardous components of the reaction medium possibly or freeing it of the organic solvent has hitherto not been solved. Furthermore, these components can be materials of value whose recovery and work-up is economically advantageous. The catalyst is usually merely concentrated but not in any way prepared for subsequent disposal or work-up, i.e. it is not conditioned. Furthermore, the concentrated catalyst suspension has, in the previous methods, usually been recirculated to a reservoir or directly to the reactor and can therefore not be isolated. Continuous operation of the reactor with simultaneous catalyst discharge and conditioning is impossible as a result.